The present invention relates to a crystalline form of the antiviral compound 5,6-dichloro-2-(isopropylamino)-1-(xcex2-L-ribofuranosyl)-1H-benzimidazole (also known as 1263W94; the compound of formula (I)), pharmaceutical formulations comprising this crystalline form of the antiviral compound, and their use in therapy.
5,6-Dichloro-2-(isopropylamino)-1-(xcex2-L-ribofuranosyl)-1H-benzimidazole is a benzimidazole derivative useful in medical therapy. WO96/01833 discloses the compound of formula (I) and its use for the treatment or prophylaxis of viral infections such as those caused by herpes viruses. The compound as disclosed in WO96/01833 is in the form of an amorphous, non-crystalline material.
The structure of 5,6-dichloro-2-(isopropylamino)-1-(xcex2-L-ribofuranosyl)-1H-benzimidazole, the compound of formula (I), is shown below: 
We have now found that the compound of formula (I) can exist in various crystalline forms and solvates. Moreover we have discovered a particular crystalline form of the compound of formula (I) Form VI, which is anhydrous and crystalline and which surprisingly has particularly good pharmaceutical properties. Form VI is the most thermodynamically stable form of the compound of formula (I). It may be easily prepared and may be manufactured on a commercial scale. It is particularly stable and essentially non-hygroscopic. Batches of this crystalline form can be consistently made to a high crystal form purity i.e. where the proportion of other amorphous and crystalline forms of the compound of formula (I) is limited. Furthermore this anhydrous crystalline form has good storage properties and can be readily formulated into pharmaceutical compositions such as tablets and capsules. The crystalline forms and solvates of the compound of formula (I) may be characterized by their X-ray powder diffraction patterns.